Signaling system



July 22, 1941.

F. A. HUBBARD 2,250,100

SIGNALING SYSTEM Filed Nov. 14, 1939 4 Sheets-Sheet l SUB SET HARMONIC GENERATOR raw/m: l2 oswcs llllijl j INVENTOA EA HUBBARD ATTORNEY July 22, 1941.

F. A. HUBBARD SIGNALING SYSTEM Filed Nov. 14, 1939 INVEN 70/? By EAHUBBARD A T TOR/V5 V July 22, 1941. F. A. HUBBARD 2,250,100

SIGNALING SYSTEM Filed Nov. 14, 1939 4 Sheets-Sheet 5 FIG. 5

INVENTOR By FA. HUBBARD A T TORNE V July 22, 1941.

F. A. HUBBARD 2,250,100

SIGNALING SYSTEM Filed Nov. 14, 1939 4 Sheets-Sheet 4 FIG. 6"

//V [/5 N TOR BVFAHUBBARD Patented July 22, 1941 SIGNALING SYSTEM Francis A. Hubbard, Maplewood, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application November 14, 1939, Serial No. 304,309

9 Claims.

This invention relates to signaling and communication systems and particularly to telephone designation transmitters.

The objects of the invention are to simplify the acts required of telephone subscribers and operators in initiating automatic telephone calls; to expedite the transmission of telephone designations from one point to another and otherwise to improve automatic telephone systems.

Systems have been proposed heretofore in which the designations of called lines and the like are transmitted from one point to another by means of alternating currents of different frequencies, which singly or in combinations, serve to characterize the different letters and numerals of which the designation is composed. In such systems it is usual practice to provide the subscribers station with a harmonic generator and with means for selecting the generated harmonies in various combinations for transmission thereat.

It also has been proposed heretofore to initially transmit the output of the harmonic generator to the central office and then through the medium erated frequencies are suppressed in combinations, each combination characterizing the designation symbol corresponding to the key depressed.

The invention will be readily understood from i the following description made with reference to to a central office to control automatic switches of a frequency suppression mechanism to suppress certain of the harmonics in combinations, thereby effecting a change in harmonic content which serves to register at the central oflice the designation letter or numeral which is characterized by the particular combination of harmonics suppressed. Such a system is disclosed in Patent No. 2,201,296, issued May 21, 1940, to A. A. Lundstrom and E. L. Norton.

According to the present invention, advantages over systems of the prior art are secured by impressing the output of the harmonic generator upon a coil which functions to drive a plurality of vibratile members or reeds, each of which is designed to vibrate at a particular resonant frequency corresponding to one of the frequencies 0 or harmonics developed by the generator. The impedance of the coil at frequencies corresponding to the resonant frequencies of the vibratile members is such when the reeds are free to vibrate that the generated harmonics are transmitted to the central office with little loss. However, when the vibrating members are damped, the impedance of the coil at frequencies corresponding to the resonant frequencies of the 0 damped members is altered so that such fre-= the accompanying drawings. in which:

Fig. 1 constitutes a diagrammatic representation, in skeletonized form, of the essential equipment employed at a subscribers station and at the central office and illustrates at a glance the general operation of the system embodying the features of the invention. In this figure the direction of a fundamental frequency current from a source [4, located at the central office, is indicated by the full arrowheads whereas the path for the frequencies generated by the harmonic generator H, located at the subscribers station is indicated by the open-head arrows;

Fig. 2 is an enlarged perspective view of the designation sender with which each subscribers station is equipped and by virtue of which any desired designation may be transmitted to the central office to control automatic switches employed in extending connections from a calling station to a called station;

Fig. 3 is a block diagram illustrating the manner in which certain of the drawings are to be assembled to effect a complete system;

Fig. 4 is a diagrammatic representation of the equipment located at a subscribers station; and

Figs. 5 and 6 illustrate the central oiiice circuits and equipment which are controlled by the designation sender at the subscribers station.

Before entering into a detailed description of the subscribers designation sender and of the system as a whole, a brief description will be made of the general operating features of the system and for this purpose particular reference is made to Fig. 1.

At subscriber's station there are located the usual telephone set indicated by the block [0, a harmonic generator II and. a keyboard sender consisting of ten keys and a tuning device l2. The telephone line extends to a central ofiice, terminating thereat in the terminals of automatic switches indicated by the numeral [3. At the central oflice there are located a source M of a fundamental frequency current and a plurality of frequency responsive circuits, such as !5, H6 and H, which selectively respond to harmonics generated by the device H in such a manner as to control the operation of automatic switches at the central office and cause them to extend a call to a called station. The operation of the system, in so far as it is disclosed in Fig. 1, is briefly as follows:

Upon initiation of a call at the calling station, the automatic switch. 13 at the central office functions in the well-known manner to seize the calling line and cause to be connected therewith the source I4, of fundamental frequency current. Current from this source traverses the path indicated by the full-head arrows and activates the harmonic generator II at the subscribers station, causing it to generate and reflect back into the line the harmonics of the fundamental frequency. Certain of these frequencies are received at the central office by the frequency selecting circuits l5, l6 and H. The designation sender at the subscribers station is then actuated in accordance with a particular designation to suppress certain of the harmonics normally received by the circuits l5, l5, ll, etc., whereupon certain switching, operations, characteristic of the suppressed frequencies, ensue, which result inthe extension of a call to a station whose assigned designation corresponds to that setup on bar 35 is in alignment with the next shortest reed 23, and so on. The plane of each code bar is at right angles to the plane of its corresponding reed, and the end of each bar facing the free end of a reed is cut away to provide a notch into which the reed end fits when the bar is actuated, as will be more fully described hereinafter.

Those portions of the code bars 34 to 38 inclusive, which are normally located immediately below the key levers 35, are each provided with a set of tooth-like projections 50 which extend upwardly from the upper edge of the levers. The edge of each tooth-like projection facing the spring biased end of its respective bar is inclined and constitutes a camming surface which cooperates with the under edge of the lever in causing the designation sender at the calling subscribers station.

The harmonic generator II is of the type fully described in R. C. Mathes Patent 2,164,335, issued July 4, 1939.

The subscribers designation sender illustrated in Fig. 2 consists essentially of a reed type tuning device, .a keyboard and apparatus controlled thereby which acts upon the vibratile members of the tuning device to selectively damp them in various combinations, in accordance with a particular code which is determined by the particular keys of the keyboard which are operated.

The tuning device per se comprises five vibratile reeds, 22 to 26 inclusive, which are held at one end between the abutting ends of a pair of clamping members 20 and 29 which are held together by means of the screws or bolts 21. The reeds are of different lengths and their clamped ends are in alignment so that their free ends project for dilferent lengths beyond the forward spaced ends of the clamping members 23 and 29. A coil 2| wound on a suitable hollow form is supported between the inner faces of the clamping members 20 and 23 in any suitable manner and encircles the reeds 22 to 25 inclusive at a point intermediate their ends; When the coil 2| is energized by. the harmonics generated by the device ll, thereeds 22 to 23inclusive vibrate at their respective resonant frequencies.

The. keyboard apparatus by which the vibralever comprises a long strip of .metal pivotally mounted on a stationary rod 3| and held in a normal unactuated position against the common stop 45 by means of a leaf spring 33, only one of which is visible in Fig. 2. The code bars 34 to 33 inclusive are located beneath the key levers 35 and are coordinately disposed thereto. These bars are supported for longitudinal movement in slots in the two extremities 39 and 40 of a substantially U-shaped support; Each code baris biased to the right by a separate spring 4 l in which position a shoulder 42 on each bar engages the outer face of the support end 39. The code bars are of different lengths and each is aligned with a different one of the re'eds 22 to 26 ment with the shortest reed 22; the next longest inclusive. The-longest code bar 34 is in alignthe bar to move to the left when the lever is actuated, that is, when the key end of the lever is depressed to cause the lever to pivot about the bar 3| in a direction away from the stop 45 and against the action of a leaf spring 33. The projections 59 of the bars 34 to 38 inclusive are so arranged that a camming surface of each of two code bars is engaged by the underside of a single lever 30 when the lever is actuated, thus causing the bars to move toward the free ends of the reeds in pairs. In the particular arrangement of toothlike projections disclosed, the actuation of key 5! results in the engagement of a camming surface on each of the bars 38 and 31, causing these bars to move inwardly and engaging the ends of reeds 25 and 25 respectively; key 52, when actuated, causes code bars 38 and 35 to move inwardly and engage reeds 25 and 24 respectively; key 53 moves code bars 36 and 3'! into engagement with reeds 24 and 25 respectively; key 54 moves code bars 35 and 38 into engagement with reeds 23 and 26 respectively; key 55 actuates code bars 35 and 31 which engage reeds 23 and 25' respectively; key 56 moves rods 35 and 35 into engagement with reeds 23 and 24 respectively; key'5l operates rod 35 and 38, causing them to engage reeds 22 and 26 respectively; key 53 causes the operation of rods 34 and 33, which in turn engage reeds 22 and 24 respectively; key 59 when actuated causes rods 34 and 35 to engage reeds 22 and 23, respectively; and key 60 operates to move rods 34 and 37 into engagement with reeds 22 and 25 respectively. It is apparent from the foregoing, that if the five reeds 22 to 26 inclusive are vibratedrat their respective reasonant frequencies, they may be damped in combinations of twos by the actuation of the key levers 30.

When the coil 2! is energized in a manner to be more fully explained hereinafter, the tuned reeds 22 to 26 inclusive will be driven at their respective resonant frequencies when the key levers 30 are in their normal unoperated positions. It is a known fact that a coil which drives a resonant armature presents at the resonant frequency a much higher electrical impedance if the armature is free to vibrate than if the armature is damped. With the key levers in their normal positions, all the reeds 22 to 26 inclusive normally would be free to vibrate when the coil 21 is energized, and the impedance of the coil would be high at all the corresponding frequencies so that the harmonics used for signaling would be transmitted over the wires 62 tude of these harmonics sent out on the line would be reduced. Thus the frequencies corresponding to the resonant frequencies of the damped reeds would be effectively suppressed. This change in harmonic content serves to register, at the central ofiice, the number of the key that was depressed, as will be more fully explained later.

The operation of the system involving the designation sender shown in Fig. 2 which functions to supress, simultaneously, two frequencies for each digit of a telephone designation, will now be described and for this purpose it will be assumed that the subscriber at station A (Fig. 4) desires to call the subscriber at station B (Fig. 5). It will also be assumed that the designation of substation B is SO-5-4321.

In accordance with usual practice the subscriber at station A removes the handset from its cradle mounting to initiate the call. The subscribers line loop is accordingly closed in the well-known manner and the line finder F (Fig. 5) starts to operate and seizes the calling line L in the usual manner. At the same time the sender selector S-l becomes effective and extends the calling line through an idle sender. When these operations have been completed, a circuit is closed from battery through the winding of line relay I 09 in the sender, over brush H0, contact of spring III on a controlling sequence switch, thence over the middle brush and corresponding terminal of finder F, over conductor I2I of the subscribers line L, through the normal closed contact of key K, through the switchhook contacts at station A, returning over conductor I 30, through the uppermost terminal and corresponding brush of finder F and sequence switch contacts H2, brush H3 of sender selector switch 8-1 to ground in the sender. The sequence switch referred to herein may be of the type well known in automatic telephone systems of the kind in which panel type selector switches are used. The line relay I09 operates in the circuit just traced and closes an obvious circuit for the slow-release relay II I. At the same time the 100 cycle tone is sent from source I4 to notify the calling subscriber that the central office equipment, is ready to receive the desired designation.

It will be noted that the key K at subscribers station A is provided with make-before-break contacts and normally holds the harmonic generator II open and the subscribers talking circuit closed on the line L. The subscriber at station A, upon receipt of the dial tone, actuates the key K to transfer the line L from the talking circuit to the harmonic generator I I and coil 2| of the designation sender. During the signal transmitting period the direct current path is maintained through coil 2|.

The harmonic generator, indicated by the numeral II, is activated by the base frequency current from source I4 to generate harmonics of the base frequency (100 cycles) and reflect them back into the line circuit L. These harmonics travel back to the central office over line L, through the base frequency elimination filter IIB, shielded transformer III, through the circuit X and thence through the primary winding sections of transformer H8. Associated with each section of the primary winding of transformer I I8 is a frequency receiving channel consisting of a secondary winding of transformer H8, a series tuned circuit, such as H9, a single vacuum tube, such as I20, with feedback through the copper-oxide units, such as I2I, and a relay, such as I22. The operation of the circuit just described is briefly as follows: Initially, the vacuum tube .is biased nearly to cut-off, a small received signal is amplified and fed back through the condenser I23 and the copper-oxide units I2I to charge the condenser I24. This condenser is poled to reduce the negative bias on the grid of the tube I20 with the result that the gain of the tube is increased and the potential on the grid further increased. The final result is that a relatively feeble signal is sufficient to produce a sharp rise in plate current through the relay I22. The sensitivity of the individual channels is adjusted by means of the variable resistance I25.

This method controls the sensitivity by changing the alternating current feedback and does not affect the tuning. It will be notedthat the selectivity of each channel is accomplished entirely by a single tuned circuit per channel. The tuning may be made as sharp as desired since the received frequency is always an exact multiple of the carrier or base frequency which may be accurately controlled at the central office.

From the immediately preceding description, it will be apparent that each relay, such as relay I22, in each channel will respond to a single harmonic with a relatively high discrimination against the others. In the present description, it will be assumed that the first channel is tuned to a frequency of 500 cycles; the second channel to a frequency of 700 cycles; :the third to 900 cycles; the fourth to 1100 cycles and the fifth to 1300 cycles.

Hereinbefore it was described how the harmonics of the carrier frequency cycles) are generated by the device H at the subscribers station and reflected back into the line circuit to eventually traverse the windings of transformer I I8 at the central office. The frequencies 500, 700, 900, 1100 and 1300 cycles are accordingly received by their respective receiving channels to cause the operation of the corresponding channel relays I22, I32, I2, I52 and I52.

It will be noted that relay I33 is normally held operated by way of the left back contacts of relays l22, I32, etc., so that prior to the receipt of the generated harmonics, this relay is in its operated position. When the generated harmonies are received by the individual frequency receiving channels, and relays I22, I32, etc., and their associated relays I26, I36, etc., operate, the circuit for relay I33 is opened so that this relay now releases its armature. In attracting their innermost armatures, relays I25, I36, I46, I56 and I66 complete an operating circuit for relay I55 which relay operates and completes an obvious circuit for relay 300, Fig. 6. Relay 300 operates and connects the register operating conductors 3II| to the relays of the first register A. In attracting its right armature, relay 300 looks under'control of relay 302 independently of relay I55. The register A is now ready to receive the first character of the selected designation.

The frequencies generated by the device I! at the subscribers station are also impressed upon the coil 2| of the designation sender. The reeds 22, 23, 24, 25 and 25 of the sender are designed to have the natural periods of vibration of 500, 700, 900, 1100 and 1300 cycles, respectively, so that in response to the receipt of these frequencies by the coil 2|, the said reeds are caused to vibrate. As previously stated, the impedance of the coil 2| at frequencies corresponding to the resonant frequencies of the vibratile members 22 to 26,

inclusive, is such that, when the members are free to vibrate, that is, undainped, the generated harmonics are transmitted to the central ofice with little loss. However, when the vibrating members are damped, the impedance of the coil at the frequencies corresponding to the resonant frequencies of the damped members is altered so that such frequencies are effectively suppressed. In accordance with the assumption that the desired designation is 550-5-4321, the calling subscriber actuates corresponding keys of the designation sender in a manner which will be now described.

Key 51 which bears the characters PR5? is actuated, causing the shaft or lever 33 associated therewith to rotate or pivot about the bar 3i against the action of a leaf spring 33. In so doing, the under edge of the lever engages the inclined edge of the innermost tooth-like projection on bar 33 and also the inclined edge of the first tooth-like projection of bar 38 andby virtue of the ensuing camming action causes the bars 3 8 and 38 to be moved inwardly, so that the vibrating reeds 22 and 23 are engaged by the notched end of the bars 34 and 38, respectively, and are damped. The impedance of the coil 21 at the frequencies 5G0 and i300 cycles is accordingly altered and the corresponding frequencies generated by the device H are effectively suppressed. Thus, the frequencies 500 and 1300 cycles, which characterize the symbol S of the desired designation are simultaneously suppressed with the result that relays I22 and IE2 at the central office are released. These relays thereupon cause the release of relays I26 and IE6 respectively, which in turn cause relay I55 to release. A circuit is now established extending from ground at the armature and back contact of relay I33, armature and back contact of relay I23, cuter armature and front contact of relay l33, third armature and front contact of relay I45, fourth armature and front contact of relay I 56, fifth armature and back contact of relay I55, conductor hi3, winding of relay lit, to battery and ground. Relay IiB operates in this circuit, locks to ground at the outer armature and back contact of relay I55 and at its right armatures connects ground potential to two of the register controlling conductors 33!, which potential passes by way of corresponding contacts and armatures of relay 333 to cause the operation of register relays 333 and 353 by way of their left windings;

Register relays 353 and 3 l3 operate and establish locking circuits for themselves extending from ground, battery, right windings and right armatures and front contacts of relays 333 and 353, conductor 38%, winding of relay 382, conductor to ground by way of the back contact and armature of relay E34. Relay 392 operates in this circuit and, at its left armature, opens the holding circuit for relay 3% which relay releases its armatures, disconnecting the register A from the register control conductors 33L At its inner right armature, relay 332 establishes a locking circuit for itself and for the operated register relays #333 and 3H3. At its outer right armature, relay 332 prepares an operating circuit for relay 335 which extends by way of the back contact and inner left armature of relay 3I3, outer right armature and front contact of relay 332 to the conductor 333 which is now disconnected from the conductor 33 at the inner armature and back contact of stepping relay I55 whose circuit was opened when relays I22 and I52 released and opened the circuits to relays I26 and I63, respectively.

The operated register relays 333 and 3!!! are now locked in the circuit including the winding of relay 332 as above indicated and serve to store the designation digit 7, or in the case described, the character S for subsequent use in controlling the automatic switches shown in Fig. 5 in a manner well known in the art and as described in Patent 1,862,549 issued June 14, 1932, to Raymond et 2.1.

When the key 5i is released by the calling subscriber the associated lever 33 is restored to normal by a leaf spring 33 and the reed damping bars 34 and 38 are restored to normal by the action of their respective springs 4|. Reeds 22 and 26 therefore are free to again vibrate. Thus all the harmonics, including those previously suppressed, are again received at the central office and relays I32 and IE2 are again operated. Under this condition, relay I55 is again operated and in closing its inner contacts completes the circuit to relay 335 which was partially completed by the operation of relay 332 as already described. Relay 305 accordingly operates and locks to the outer left armature of relay 3I3 and connects the register control conductors 33I to the register B for the receipt of the next digit, or character O of the desired designation. The operation of relay I55 also opens the locking circuit for relay I13 which relay thereupon releases. All relays I22, I32, M2, I52 and I62 as well as their associated relays I25, I33, I46, I56 and I66 are, therefore, operated between digits and, except for the stepping action effected by relay I55, perform no useful function at this time.

The actuation of key 5'! by the calling subscriber is followed by the actuation of key 58 which bears the characters MNOG. The lever 30 associated with the key 55 accordingly pivots about the bar 3i, causing its own under edge to engage the inclined edges of the second toothlike projection of each of the bars 35 and 36, causing the bars 35 and 33 to move inwardly and engage the vibrating members 23 and 24, respectively. These members are accordingly damped and the corresponding frequencies 700 and 900 cycles are suppressed. Relays I32 and M2 at the central office accordingly release and, in restoring their right armatures, cause the release of relays I36 and I43. The release of relays I36 and I65 causes the circuit for stepping relay I55 to be opened whereupon this relay restores its armatures which perform no useful function other than to prepare a locking circuit for the next relay of relays Ilil to I 79, inclusive, to be operated.

v A circuit now exists which may be traced from ground, back contact and armature of relay I 33, armature and front contact of relay I25, inner armature and back contact of relay I 36, middle armature and back contact of relay I45, conductor Itl, winding of relay H5 to battery and ground. Relay I15 operates in this circuit and locks under control of relay I55. In attracting its two right armatures, relay H5. connects ground potential to two other conductors of the register control conductors 33I and thence by way of corresponding left contacts of relay 305, to the left windings of register relays 3H and 3I2 of register B. These register relays operate and look in a circuit including their right windings and armatures, conductor 31", winding of relay 3E3, conductor 338 to ground at the armature and back contact of relay I3I. Relay 3I3 operates in this circuit and at its outer left armature and back contact opens the locking circuit to relay 305 which relay restores its armatures. In releasing its left armatures, relay 305 disconnects the register B from the register control conductors 30 I. Relay (H3, at its inner right armature, establishes a locking circuit for itself and at its outer right armature prepares an operating circuit for relay 300. The register relays 3H and 3I2 are now locked and store the character of the selected designation SO-- 1321.

The key 55 at the subscribers station is released and relays I 32 and I 42 which were released for the transmission of the second digit of the telephone designation are again operated. Corresponding relays I35 and I46 are operated and the circuit to stepping relay I55 again closed. The closure of the inner contacts of relay I55 completes the energizing circuit for relay 305 which was prepared by the operation of relay 313 as previously described. Relay 305 accordingly operates and locks to the outer left armature of relay 3I8. At its left armatures and front contacts, relay 306 connects the register control conductors 31H to the register C for the receipt of the next digit 5 of the desired designation.

The subscriber now actuates key 55 which causes bars 35 and 31 to move inwardly, in a manner now apparent, and to damp the reeds 23 and front contact of relay I26, inner armature and back contact of relay I35, middle armature and front contact of relay I 45, third armature and back contact of relay I56, conductor I54, winding of relay I14 to battery and ground. Relay I14 operates in this circuit and at its right armature connects ground to another conductor of the register control conductors I, which passes over a corresponding front contact and left armature of relay 305 and thence to battery i and ground through the left winding of register relay 3I'I of register C. Relay 3I1 accordingly operates and locks in a manner similar to that described in connection with the now locked-up relays of registers A and B. Relay 1H8 operates r as a result of the operation of relay 3 I 1 and opens the locking circuit to relay 3% and establishes a locking circuit for itself. Relay 306 in releasing its left armatures disconnects the register control conductors 30I from register C. Relay I 1 3 operated, locks to the outer back contact of relay I55.

In Fig. 6, only four registers are illustrated, A, B and C for the office codes and U for the units digits. It is to be understood that the thousands, hundreds, and tens registers are included in the complete system. In attracting its outer right armature, relay 3I8 prepares an operating circuit for the next succeeding relay of the thousands register, such as relays 300, 305 and 305 of registers A, B and C,'and in attracting its inner right armature relay SIB locks in an obvious circuit with the register relay 3I1 of register C.

It is believed unnecessary to describe in detail the manner in which the remaining digits 4, 3, 2, 1 of the desired designation are transmitted to the central ofiice and stored in the registers thereat since such operations are now apparent from the previous descriptions having to do with the transmission of the designation characters S, 0 and 5. Suffice it to say that keys 54, 53, 52 and 5| are successively actuated by the calling subscriber whereupon bars and 38, 35 and 31, 35 and 38, and 31 and 38 are successively moved inwardly in pairs, causing the successive damping of the following groups of reeds: 23 and 26, 24.and 25, 24 and 26, and 25 and 23. The frequencies 700 and 1300 cycles are thus suppressed for the digit "4; 900 and 1100 cycles for the digit 3; 900 and 1300 cycles for the digit 2 and 1100 and 1300 cycles for the digit 1. The suppression of these frequency combinations causes corresponding relays of the group of relays I22, I32, I42, I52 and I52 to restore their armatures to control the operation of corresponding relays I10 to I19, inclusive, in a manner which is now evident. The circuit arrangement is such that relay I10 operates on the suppression of the 1100 and 1300 cycle frequencies caused by the actuation of key 5|; relay I1I operates on the suppression of 900 and 1300 cycle frequencies caused by the actuation of key 52; relay I12 operates on the suppression of 900 and 1100 cycle frequencies caused by the actuation of key 53; relay I13 operates on the suppression of 700 and 1300 cycle frequencies caused by the actuation of key 54; relay I 14 operates on the suppression of 700 and 1 100 cycle frequencies caused by the actuation of key 55; relay I15 operates on the suppression of 700 and 900 cycles when key 56 is actuated; relay I16 operates when key 51 is actuated to suppress the 500 and 1300 cycle frequencies; relay I11 operates when the frequencies 500 and 900 cycles are suppressed by the actuation of key 58; relay I18 operates when frequencies 500 and 700 cycles are suppressed when key 59 is actuated; and relay I19 operates when key 60 is actuated to suppress the frequencies 500 and 1 cycles. a

It has already been described how the operation of relays I16, I15 and I14 results in the operation of register relays 303 and 3H] of register A to store the digit '1, or character S, of register relays II I and H2 of register B to store the digit 6, or character 0, and to relay M1 to register C to store the digit 5 of the desired designation. Relay I13 causes the opera-tion of a relay in the thousands register to store the digit 4; relay I12 causes the operation of two relays in the hundreds register which correspond to'the relays 3H and 322 of the B register to store digit 3; relay I II causes the operation of a relay in the ten-s register corresponding to relay 303 of'the A register to store digit 2; and When relay I10 operates, relay 331 in the units register U will operate and lock to store the units digit 1.

Due to the successive operation of keys 5!. to 51, inclusive, of the designation send-er, the desired designation SO-5- l, 3, 2, 1 is stored in the central office registers.

After the operation of the last key, that is, key 5I in the case described, the calling subscriber restores the key K to normal to. transfer the loop from the subscribers sender to the talking set.

It will be understood that after the registra-s tion of each digit in its particular register, the stepping relay I55 functions to cause the register control conductors 30! to be connected to the next succeeding register as described in connection with the operation of such relays 305 and 396. This stepping function is the immediate rei sult of the release of the designation sender keys to 59, inclusive, which results in the transmission of all the generated harmonics, including those suppressed when the keys are operated, to the, central office.

It will be notedthat the foregoing description is confined to a system in which each character or digit of a telephone designation is characterized by two frequencies suppressed simultaneously. It is apparent that the designation sender will function as well in a system wherein the designation characters are characterized by other than two frequencies. It is also within the scope of applicant's invention to connect the coil 2| in series with the line instead of in shunt therewith, in which case the reeds would normally all be damped and the keys would function to selectively remove the dampers.

As shown in Fig. 5 an auxiliary control channel V is added to the signal receiving channels and is a duplicate thereof. This channel is made selectively responsive to a frequency just above the frequencies utilized for designation transmission. A frequency of 1500 cycles is suitable in the case described.

With the receiving apparatus used on a short loop, it is obvious that a very high degree of suppression to the harmonics would be necessary at the subscribers set in order to reduce them well below the level normally received on long loops. To avoid this, the extra harmonic receiving channel V is-used which is not affected by the signaling processes and serves only to control the input to the apparatus. As above stated, the vol- .ume control channel is a duplicate of the signaling channels and is placed just above them in frequency. The change in plate current is used to change the resistance of a copper oxide shunt circuit. of two 700-ohm resistances and two -microfarad condensers in one of the two series arms and four one-quarter inch discs of copper oxide connected toeffect a T network as shown. The two condensers referred to as well as the condenser in the other series arms are blocking condensers.

At'low levels the direct current through the copper oxide discs is low and the copper oxide resistance is high. Th circuit X will therefore cause little loss, but as the level increases, the direct current will increase sharply, reducing the resistance of the copper oxide units and accordingly increasing the loss and thereby reducing the level at which the transmitted signals are received in their respective receiving channels. In'this manner the transmitted signals are received at asubstantially constant level irrespective of the length of line over which they are transmitted.

V In the particular embcdiment'of the invention disclosed in the drawings, the vibrating reeds are energized by a base frequency supplied from the central office. It will be apparent to those skilled in'the art that the fundamental scheme for alternating current dialing is capable of variations which come within the scope of the present in- A suitable input circuit X may consist vention. A particular variation which readily suggests itself is one in which the tuned reeds may be used as coupling members between the driving coil, fed from the harmonic generator, and a carbon button or other alternating current generator so that the output of the carbon button will contain only those harmonics represented by the reeds which are free to vibrate. In such a Variation and with no keys of the designation sender depressed, all the reeds are free to vibrate and the output of the carbon button will contain all five harmonics corresponding to the five vibrating reeds. Each key when depressed damps two of the reeds thereby suppressing two corresponding harmonics. Thus the designation signals reach the central office in exactly the same form described in connection with the system disclosed in the drawings.

What is claimed is:

l. The combination in a signaling system of a line, a harmonic generator responsive to current of a base frequency on said line for reflecting harmonics of the base frequency into said line for transmission thereon, and means for suppressing certain of the generated harmonics comprising a plurality of vibratile reeds each having a natural period of vibration corresponding to a particular one of the generated harmonics, an impedance coil responsive to the generated harmonics for vibrating said reeds, said coil presenting an impedance during the vibration of said reeds such that the harmonics corresponding to the natural period of the vibrating reeds are transmitted over said line with no appreciable loss and means for damping said reeds whereby said coil presents an impedance at frequencies corresponding to the natural periods of the damped reeds such that the corresponding harmonics are suppressed.

2. The combination in a signaling system of a line, a harmonic generator responsive to current of a base frequency on said line for reflecting harmonics of the base frequency into said line for transmission'thereover, and means for selectively suppressing certain of the generated harmonics in pairs comprising a plurality of vibratile reeds each having a natural period of vibration corresponding to a particular one of the generated harmonics, an impedance coil responsive to the generated harmonics for vibrating said reeds, said coil presenting an impedance during the vibration of said reeds such that the generated harmonics are transmitted over said line with no appreciable loss and means for selectively damping said reeds whereby sail coil presents an impedance at frequencies corresponding to the natural periods of the damped reeds such that the corresponding harmonics are suppressed.

3. The combination in a signaling system of a line having a station thereon, a source of alternating current of a base frequency, mean for connecting said source of said line, a multifrequency generator at said station responsive to current from said source for generating harmonies of the base frequency and reflecting them into said line, and means for controlling the transmission of certain of the generated harmonies over said line comprising an impedance coil connected in said line whose impedanc at the'frequencies of said certain harmonics controls the transmission thereof over said line, and means for varying the impedance of said coil at the frequencies of said certain harmonics comprising a plurality of vibratile members driven by said coil and each tuned to vibrate at a frequency corresponding to a particular one of said certain harmonics, and means for selectively controlling th vibration of said vibrating members.

4. The combination in a signaling system of a line, a source of alternating current of a base frequency, means for connecting said source to said line, a hormonic generator responsive to current from said source for generating harmonics of the base frequency and reflecting them into said line for transmission thereover, a plurality of vibratile members each tuned to vibrate at a particular resonant frequency, a driving coil therefor connected to said line responsive to the generated harmonics for vibrating said vibratile members at their respective resonant frequencies, and means for selectively damping the vibrating members in various combinations to cause said coil to present an impedance at the resonant frequencies of the damped members such that the corresponding generated harmonics are excluded from said line.

5. The combination in a signaling system of a line having a station thereon, a source of alternating current of a given frequency at a central office, means responsive to the initiation of a call at said station for connecting said source to said line, a harmonic generator at said station responsive to current from said source for generating harmonics of the given frequency, a tuned reed for each of a given number of the generated harmonics, an impedance coil responsive to the generated harmonics for vibrating said reeds and means at said station for damping said reeds in various combinations to cause the impedance of said coil at frequencies corresponding to the natural periods of the damped reeds to be altered whereby the corresponding generated harmonics are suppressed.

6. The combination in a signaling system of a line having a station thereon, a source of alternating current of a given frequency at a central office, means responsive to the initiation of a call at said station for connecting said source to said line, a harmonic generator at said station responsive to current from said source for generating harmonics of the given frequency and transmitting them over said line, translating devices at said central office selectively operating in response to the transmission of certain of the generated harmonics over said line, a tuned reed at said station for each of said certain harmonies, an impedance coil responsive to the generated harmonics for vibrating said reeds and means at said station for selectively damping said reeds in pairs to cause the impedance of said coil at frequencies corresponding to the natural periods of the damped reeds to be altered, whereby the corresponding generated harmonies are suppressed in pairs and corresponding pairs of said translating devices are released.

7. The combination in a signaling system of a line, a source of alternating current of a base frequency, means for connecting said source to said line, a harmonic generator responsive to current from said source for generating harmonics of the base frequency and reflecting them into said line for transmission thereover, and means for excluding certain of the generated harmonics from said line comprising a plurality of distinctively tuned vibratile members having natural periods of vibration corresponding to said certain of the generated harmonics, an impedance coil subject to the generated harmonics for driving said vibratile members and presenting an impedance at frequencies corresponding to the natural periods of said vibratile members when driven by said coil such that the corresponding generated harmonics are transmitted over said line with no appreciable loss and means for selectively damping said vibratile members whereby the impedance of said coil at frequencies corresponding to the natural periods of the damped members is altered and the corresponding generated harmonics consequently excluded from said line.

8. The combination in a signaling system of a line, a multifrequency generator for transmitting a plurality of different frequencies over said line, and means for selectively excluding certain of the generated frequencies from said line comprising a plurality of distinctively tuned reeds, a driving coil therefor connected to said line, and responsive to current from said generator and means for selectively damping said reeds to cause said coil to present an impedance at frequencies corresponding to the vibrating frequencies of the damped reeds such that the corresponding generated frequencies are excluded from said line.

9. The combination in a telephone signaling system of a line circuit, a station thereon, a source of alternating current of a base frequency, means responsive to the initiation of a call at said station for connecting said source of current to said line circuit, means at said station responsive to current fromsaid source for generating harmonics of the base frequency in said line circuit, translating devices operated in response to the generation of harmonics at said station, and a designation transmitter at said station comprising a tuned reed for each of a certain number of the generated harmonics, an impedance coil responsive to the generated harmonics for vibrating said reeds, said coil, during the vibration of said reeds, presenting an impedance to the transmission of said certain number of the generated harmonics such that they are transmitted over said line without loss and means for selectively damping said reeds in combinations characteristic of the digits of the telephone designation to cause the impedance of said coil at frequencies corresponding to the natural period of the damped tuned reeds to be altered, whereby corresponding harmonics are suppressed and corresponding translating devices released and means controlled by said translating devices for registering the designation.

FRANCIS A. HUBBARD. 

